Answer
The order of stability is shown in the attached image.
Work Step by Step
The stability of the conjugate base depends on if it is charged or not, and if it is charged, the positive charge should not be on a highly electronegative element. With these considerations, we see that :
$NH_3$ is the most stable conjugate base because it is neutral, and the polarity of $N-H$ bonds is lesser than that of the $O-H$ bonds in $H_2O$, which is the next most stable base. This is because the electronegativity of nitrogen is less than oxygen.
Then, the further order of stability decreases as the positive charge is first on nitrogen (electronegative) and then on oxygen (more electronegative than oxygen). Out of the last two compounds, the former is resonance stabilised. Hence, the order of stability is as depicted in the image.
